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Post Info TOPIC: massive stars


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Massive blue stars
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They're big, fat, blue and hot.
These blue stars are the brightest stars in the galaxy, and they are destined to die young, having life spans of "only" a couple of million years. Once their time is at an end, they provide anyone watching with the spectacular show of a super nova.
These stars are also rare.
Alma College physics professor Cameron Reed is putting them all in a catalogue.
He's got about 20,000 in his database so far. And although 20,000 may not sound rare, astronomers don't know exactly how many stars there are in the Milky Way, but estimates range upward of 100 billion. There may be as many as 200,000 blue stars and they are 10 to 60 times the size of the sun.

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RE: massive stars
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Title: Massive stars: Feedback effects in the local universe
Authors: M.S. Oey (U. Michigan), C.J. Clarke (IoA, Cambridge)

We examine self-consistent parameterisations of the high-mass stellar population and resulting feedback, including mechanical, radiative, and chemical feedback, as we understand them locally. To date, it appears that the massive star population follows simple power-law clustering that extends down to individual field OB stars, and the robust stellar IMF seems to have a constant upper-mass limit. These properties result in specific patterns in the HII region LF and ionisation of the diffuse, warm ionised medium. The resulting SNe generate superbubbles whose size distribution is also described by a simple power law, and from which a galaxy's porosity parameter is easily derived. A critical star-formation threshold can then be estimated, above which the escape of Lyman continuum photons, hot gas, and nucleosynthetic products is predicted. A first comparison with a large H-alpha sample of galaxies is broadly consistent with this prediction, and suggests that ionising photons likely escapes from starburst galaxies. The superbubble size distribution also offers a basis for a Simple Inhomogeneous Model for galactic chemical evolution, which is especially relevant to metal-poor systems and instantaneous metallicity distributions. This model offers an alternative interpretation of the Galactic halo metallicity distribution and emphasizes the relative importance of star-formation intensity, in addition to age, in a system's evolution. The fraction of zero-metallicity, Population III stars is easily predicted for any such model. We emphasise that all these phenomena can be modelled in a simple, analytic framework over an extreme range in scale, offering powerful tools for understanding the role of massive stars in the cosmos.

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NGC3576
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NGC 3576 is a giant HII region of glowing gas located about 9,000 light years from Earth in the constellation Carina. In the Chandra image of this star forming region, lower-energy X-rays (0.5-2.0 keV) are shown in red and higher-energy X-rays (2-8 keV) are in blue. Chandra reveals a cluster of point-like X-ray sources, some of which are massive young stars that are shredding the cloud of gas from which they formed. The blue sources are stars that are deeply embedded in gas. Regions of diffuse X-ray emission are likely caused by hot winds flowing away from the most massive stars. Some of the diffuse gas near the centre of the image is also deeply embedded.

ngc3576b
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Credit NASA

Coordinates (J2000) RA 11h 11m 53.80s | Dec -61' 18" 25.00

HII (pronounced "H-two") regions are where stars are born from condensing clouds of hydrogen gas (they are named for the large amounts of ionised atomic hydrogen they contain.) These regions are characterized by hot, young, massive stars which emit large amounts of ultraviolet light and ionise the nebula. Because NGC 3576 is very dense, many of the young, massive stars visible in the Chandra image have previously been hidden from view. A cluster of stars is visible in infrared observations, but not enough young, massive stars have been identified to explain the brightness of the nebula. Astronomers have found a large flow of ionised gas in radio observations and huge bubbles in optical images that extend out from the edge of the HII region. Taken with the X-ray data, this information hints that powerful winds are emerging from this hidden cluster.

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RE: massive stars
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Observations confirmed a leading theory that a doughnut-shaped ring of material could be responsible for the formation of massive stars, scientists reported Wednesday.

Smaller stars typically form when clouds of dust and gas collapse into a ball of compact material.
Stars that are 10 times more massive than the sun, however, generate powerful stellar radiation, which can prevent the accumulation of material.

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The most massive stars in our galaxy weigh as much as 100 small stars like the Sun. How do such monsters form?
Do they grow rapidly by swallowing smaller protostars within crowded star-forming regions?
Some astronomers thought so, but a new discovery suggests instead that massive stars develop through the gravitational collapse of a dense core in an interstellar gas cloud via processes similar to the formation of low mass stars.

"In the past, theorists have had trouble modelling the formation of high-mass stars and there has been an ongoing debate between the merger versus the accretion scenarios. We've found a clear example of an accretion disk around a high-mass protostar, which supports the latter while providing important observational constraints to the theoretical models" - Nimesh Patel, astronomer at the Harvard-Smithsonian Centre for Astrophysics (CfA).

IMAGE (69kb, 576 x 720)

Patel and his colleagues studied a young protostar 15 times more massive than the Sun, located more than 2,000 light-years away in the constellation Cepheus. They discovered a flattened disk of material orbiting the protostar. The disk contains 1 to 8 times as much gas as the Sun and extends outward for more than 30 billion miles - eight times farther than Pluto's orbit.

The existence of this disk provides clear evidence of gravitational collapse, the same gradual process that built the Sun. A disk forms when a spinning gas cloud contracts, growing denser and more compact. The angular momentum of the spinning material forces it into a disk shape. The planets in our solar system formed from such a disk 4.5 billion years ago.

Evidence in favour of high-mass accretion has been elusive since massive stars are rare and evolve quickly, making them tough to find. Patel and his colleagues solved this problem using the Submillimetre Array (SMA) telescope in Hawaii, which offers much sharper and highly sensitive imaging capabilities compared to single-dish Submillimetre telescopes.
SMA is currently a unique instrument that makes such studies possible by allowing astronomers to directly image the dust emission at Submillimetre wavelengths and also to detect emission from highly excited molecular gas.
The team detected both molecular gas and dust in a flattened structure surrounding the massive protostar HW2 within the Cepheus A star formation region. SMA data also showed a velocity shift due to rotation, supporting the interpretation that the structure is a gravitationally bound disk.
Combined with radio observations showing a bipolar jet of ionized gas, a type of outflow often observed in association with low-mass protostars, these results support theoretical models of high-mass star formation via disk accretion rather than by the merging of several low-mass protostars.

"Merging low-mass protostars wouldn't form a circumstellar disk and a bipolar jet. Even if they had circumstellar disks and outflows before the merger, those features would be destroyed during the merger" - Salvador Curiel, co-author, National Autonomous University of Mexico (UNAM), who is on sabbatical leave at CfA.

The team plans more detailed observations using the SMA and the National Radio Astronomy Observatory's Very Large Array, which initially detected the bipolar jet.

The researchers, in addition to Patel, Ho, and Curiel, are: P. T. Ho, T. K. Sridharan, Q. Zhang, T. R. Hunter and J. M. Moran, of CfA; Jose M. Torrelles, Institute for Space Studies of Catalonia (IEEC)-Spanish Research Council (CSIC), Spain; and J. F. Gomez and G. Anglada, Instituto de Astrofisica de Andalucia (CSIC), Spain.

This research is being reported in the September 1, 2005, issue of Nature.

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-- Edited by Blobrana at 17:38, 2008-04-07

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